Stars are often referred to as early or late types. Early indicates hotter while late indicates cooler. This stems from an early 20th-century model of stellar evolution in which stars were powered by gravitational contraction via the Kelvin–Helmholtz mechanism whereby stars start their lives as very hot "early-type" stars, and then gradually cool down, evolving into "late-type″ stars. This mechanism provided ages of the Sun that were much smaller than what is observed, and was rendered obsolete by the discovery that stars are powered by nuclear fusion . 
The classification was invented by Herbert Putnam in 1897, just before he assumed the librarianship of Congress. With advice from Charles Ammi Cutter , it was influenced by his Cutter Expansive Classification , the Dewey Decimal System , and the Putnam Classification System (developed while Putnam was head librarian at the Minneapolis Public Library ).   It was designed specifically for the purposes and collection of the Library of Congress to replace the fixed location system developed by Thomas Jefferson . By the time Putnam departed from his post in 1939, all the classes except K (Law) and parts of B (Philosophy and Religion) were well developed.